Method for preheating pulverous materials prior to their introduction into a melting furnace

ABSTRACT

The present invention relates to method and apparatus for preheating pulverous materials, such as glass batch constituents, prior to their introduction into a melting furnace to increase the efficiency and output of the melting installation. The pulverous materials, including a dessicant-type constituent, are passed downwardly through a shell and tube preheater to absorb the contained moisture during their downward travel through the preheater. The inclusion of a dessicant material prevents moisture condensation and build-up of the pulverous material within the tubes, especially in cooler areas, which can cause tube pluggage. The subject invention is of particular utility to the glass industry and especially glass melting furnaces, but is also applicable to other types of furnaces.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation-in-part application of pending U.S.patent application Ser. No. 184,875, filed Sept. 8, 1980, in the name ofthe same inventor and having the same title, which application isassigned to the same common assignee as the present application.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Various types of manufacturing processes are known in which the startingmaterials are introduced into the furnace while cold or at ambienttemperature by the use of either continuously or discontinuouslyoperating devices. Such devices are frequently protected by a hydraulicor other cooling apparatus which absorbs the heat from the furnace andadditionally intensifies the cooling of the materials introduced intothe furnace. In these methods, and especially in melting glass, thestarting materials are subjected to heating only after they have beenintroduced into the furnace where they receive, at high temperature, thequantities of heat necessary for heating them, for ensuring completionof the endothermic reactions, and for imparting thereto sufficientfluidity to ensure homogenization and refining of the resultant moltenglass mass. It has been observed in glass making that the greater partof the heat delivered to the starting materials is directed toincreasing the temperature of the starting materials rather than toproducing the melting reactions. In most known methods, the startingmaterials are deposited on top of the molten bath and are subjected toradiation from the flames circulating with great turbulence above them.Since the newly-introduced materials are poor conductors of heat, theheat exchange is poor, which appreciably slows the melting process.

The present invention especially relates to increasing the efficiencyand output of glass melting installations, and provides means whereby aglass melting furnace may be operated continuously and uniformly at fullcapacity or beyond, if desired. Apparatus is provided for preheating thethoroughly-mixed, glass-forming ingredients before the same are suppliedto the melting furnace, and preferably utilizing the heat of the wastegases from the melting furnace in such preheating of the glass mixtureand causing continuous passage of the glass mixture by gravity throughthe preheater for subsequent delivery to the melting furnace.

This invention comprises an improved process and means for practicingthe process to accomplish the aforesaid objects, and in the provision ofan improved arrangement of apparatus for preheating the glass batchmixture and for utilizing waste gases from glass melting furnaces, orpreheated hot air from such furnaces, as more fully set forth in thefollowing specification, and as particularly pointed out in the appendedclaims.

The provision of the preheater for the glass-making mixture enables theutilization, for heating the same, of the heat in the waste gases fromthe melting furnace which otherwise would go to waste up the stack.While the use of hot waste gases is preferred to operate the preheater,preheated air from the furnace heat-recovery "checkers" area which isused for combustion, or a supplemental heat source such as an oil or gasburner, alone or in combination, may also be used to heat the air orwaste gases for operating the preheater. Also, atmospheric air may beheated to operate the batch preheater. The provision of the preheater,continuously delivering glass batch mixture at a proper predeterminedelevated temperature, to a melting furnace, which is used with eithercontinuous or batch processes, permits more uniform operation of thefurnace with a significant increase in efficiency of operation and inthe output of the furnace.

2. Description of Prior Art

There is a considerable number of earlier-issued U.S. patents which dealwith initially preheating the glass batch mixture prior to its deliveryinto the glass furnace. U.S. Pat. No. 3,607,170 to Malesak disclosesmethod and apparatus in which the glass mixture is preheated in anon-oxidizing atmosphere while being advanced in a given directionthrough a preheating zone of a tunnel kiln. A mixture of glass powderand foaming agent is dissolved into a hopper having a series of tubesthrough which the mixture passes.

U.S. Pat. No. 3,172,648 to Brichard relates to preheating of pulverousmaterials in which the quantity and rate of flow of the fumes in thepreheating zone are in direct contact with the glass formingingredients, such contact causing an entrainment of dust in the emittingfumes.

U.S. Pat. No. 4,045,197 to Tsai et al relates to apparatus and methodfor recovering the waste heat from the exhaust gases of a glass meltingfurnace and transferred by heat pipes to an enclosue in which incomingglass batch materials are preheated prior to being fed to a furnace formelting. The heat pipes contain metallic sodium as the working fluid.

U.S. Pat. No. 3,788,832 to Nesbitt et al, and U.S. Pat. No. 3,880,639 toBodner et al, owned by the same common assignee as the presentapplication, both relate to the preheating of agglomerated glass batchmaterials by direct contact with a gaseous effluent being exhausted froma glass melting furnace.

U.S. Pat. No. 3,185,554 to Sweo et al relates to a method of preheatingglass batch materials by independent heating means other than exhaustedeffluent so that there is no unpredictable relationship between varyingamounts of waste heat and the amount of heat provided for preheatingunmelted batch materials.

A considerable number of other patents relates to the direct heatexchange between incoming batch materials and exhaust gases from a glassmelting furnace. These U.S. Pat. Nos. are: 3,607,190 to Penberthy,4,026,691--Lovett, 3,526,492--Motsch, 3,350,213--Peyches,1,543,770--Hilbert, 3,753,743--Kukada, 1,610,377--Hitner, and4,099,953--Rondeaux. Many techniques have been disclosed in the patentliterature for direct and indirect heat exchange between hot exhaustgases from a glass melting furnace and incoming batch materials;however, none is capable of achieving the results attainable by thepresent invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view partially broken away of the glass batchpreheater apparatus for practicing the present invention.

FIG. 2 is an enlarged fragmentary vertical sectional view of thepreheater apparatus as shown in FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawings, a glass melting furnace 10 of theregenerative type having a bottom of fire brick, whereon the melt ofglass forming ingredients is deposited, is indicated schematically inFIG. 1. Gas and air are normally mixed and burned in the furnace abovethe glass forming materials, and the resulting heat melts the mixedmaterials to a mass of molten glass, which is delivered or worked fromone end following refining. The regenerative or heat-accumulatingchambers or passageways are normally located beneath the furnace meltingchamber.

The air is usually passed through the regenerative passages beneath thefurnace bottom for preheating and through side ports which lead into thefurnace melting chamber where it is mixed with fuel which is burned tomelt the glass forming materials. The hot waste gases are then passedthrough opposite side ports and then through the regenerative passagesfor heat recovery, and then to flue ducts and a discharge stack. After alimited period of operation in this manner, the path of travel of theincoming air is switched, by suitable dampers and timers, so that theair then enters the melting chamber from opposite ports, the hot wastegases then passing off through opposite flue passages and ducts to thestack. By the alternate use of the regenerative passages for incomingair and outgoing hot waste gases, the incoming air is preheated by thebricks of the regenerative passages which have been heated by theescaping flue gases which previously passed therethrough. All of theforegoing description pertains to well known glass furnace constructionand is merely set forth by way of example. The waste gases from thefurnace, the preheated combustion air from the furnace, or supplementaryatmospheric air which has been suitably preheated, may be employed tooperate the batch preheater.

In accordance with a preferred embodiment of the present invention, apreheater 11 is mounted adjacent to the batch feeding end of the glassfurnace at an elevation higher than the normal elevation of the furnacebatch chargers. The glass forming ingredients in suitably intermixedcondition are delivered to the top of the preheater 11 by any suitablemeans such as a vertical elevator 32. The vertical elevator may consistof any endless chain or bucket-type arrangement of standardconstruction, capable of taking the glass forming mixture from a pile orhopper and delivering it into a chute 13 through which it passes intothe top of the preheater 11. The glass forming mixture comprises thenormal intermixed batch constituents and may or may not contain brokencullet for forming the glass melt. The cullet, when present, normallyhas a size ranging from about 1/2 inch to 1 inch U.S. mesh size, thesmaller size being preferred for passage through the preheater toprevent bridging within or over the tubes.

The preheater typically comprises a vertical chamber 14 having arectangular cross-section with a frusto-pyramidal top cover 15. The mainmass of glass batch is delivered through a chute leading into the bottomarea of vertical elevator 32 for delivery to the top of preheater 11.Between the enclosed top cover and the main body portion of thepreheater is located an interior horizontal upper plate 16 into which aplurality of open-ended tubes 17 are headed at their upper ends. Thetubes are mounted in spaced-apart array in parallel, vertical alignmentfor passage of the glass batch therethrough. The space between the upperends of adjacent tubes is covered with a bulbous nose member 30 toensure free flow of the glass batch uniformly into each tube. One nosemember 30 is located between each array of upper ends of four adjacenttubes. If necessary, a pressurized gaseous fluid may be employed todrive the moisture and water vapor contained in the glass batchdownwardly with the flow of batch to avoid tube pluggage due tocondensation.

The tubes 17 preferably have about a 4 inch internal diameter and extendthroughout the central major portion of the preheater to an interiorhorizontal lower plate 18 into which they are similarly headed. Thus,the central portion of the preheater comprises a shell and tubearrangement. Tubes having about the stated dimension are capable ofhandling batch, including cullet, while tubes of about 2 inch internaldiameter are able to handle cullet-free batch. The lower open ends ofthe tubes extend a short distance below lower plate 18 to allow freedischarge of the glass batch therefrom. The space around thedownwardly-projecting tube ends normally remains open above thecollected batch emitting from the tubes. The number of tubes anddimensions of the preheater will depend upon the size of the glassmelting furnace with which it is employed, and the desired conditions ofuse. The tubes are mounted on about 6 to 8 inch centers where 4 inchinternal diameter tubes are employed, the corner tubes usually beingomitted where the preheater has a rectangular or square horizontalcross-section. The tubes are preferably comprised of carbon or stainlesssteel for long-term use without rusting or corrosion, and are normallyequi-spaced for optimum particulate batch flow.

The lower region of the preheater comprises a frusto-pyramidal bottomhopper 20 into which the open-ended tubes 17 deliver the heated glassbatch. The bottom hopper terminates at its lower extremity into ascrew-driven batch removal chamber 21 which interconnects with a valvemember 22. The valve member has an exit portion for directing the heatedglass batch through a chute 23 to a batch charger 24. The batch chargeris capable of delivering the heated glass batch into the furnace 10through a screw-driven feed member or other means as known in the art.

Immediately above the bottom interior header member 18 of the preheater,an incoming waste gas duct 25 is mounted for delivering hot waste gasesinto a lower region of the preheater. The duct is designed to open outinto a relatively-flat, wide duct inlet having a width comparable to thepreheater for introducing the hot gases across its full width.

Immediately below the upper interior header member 16 of the preheater,an outgoing waste gas duct 26 for removing hot waste gases from an upperregion is mounted. The duct consists of a relatively-flat, wide ductoutlet having a width comparable to the preheater for removing the hotgases across its full width.

A plurality of flat baffle plates 27 is mounted in spaced-apart,staggered relation within the preheater between the upper and lowerinterior header plates 16 and 18. The baffle plates 27 have openingstherein through which the tubes 17 extend between their upper and lowerextremities. The baffle plates are able to direct the upwardly coursinghot waste gases into a circuitous path to provide turbulence to thegases and thereby improve heat transfer to the tubes and the glass batchmoving downwardly by gravity therewithin.

The glass batch contains a dessicant material as one of the glassforming constituents. Either calcined lime or calcined dolomitic limecan be used as the moisture-absorbing batch component. It is preferredthat about one-half of the required calcium oxide content of the normalsoda-lime glass composition be introduced into the batch in calcinedform to retain the moisture normally contained in the batch duringpreheating. Thus, where the batch would normally contain raw lime, atleast a substantial portion of the lime is calcined into anhydrous ordessicating form prior to use. In one case where the batch containedburned dolomite and about 50 percent by weight cullet (of 1/2 inchscreen size), the preheater was found to work well with no pluggage oftubes. In this situation, it appeared that the water from the cullet andother sources was absorbed by the burned dolomite and was not boiled offby the heating. Also, the water appears to be held very tightly byburned lime. It is preferred that slightly more than the stoichiometricamount of glass forming dessicant material be employed to chemicallyreact with the amount of moisture present in the glass batch. Uponreaction, the Ca(OH)₂ or Ca).H₂ O will not give up its water ofhydration until it reaches a temperature of about 1076° F. (580° C.),which is above the preheating temperature of the glass batch. Thus, acalcium oxide containing glass batch is an ideal vehicle for theintroduction of the CaO in at least partly calcined form.

The batch mixture passes gradually and continuously through thepreheater by gravity from top to bottom. The moisture normally found inconventional unwetted glass batch in an amount ranging from 0.1 to 2percent by weight is vaporized due to the batch heating. The burned orcalcined lime absorbs the moisture, especially in an upper region of thepreheater, and retains the same during downward travel of the batch.Thus, its condensation or collection in the upper cooler areas of thetubes is prevented. The batch is then delivered, uniformly heated, andwell mixed, with the moisture chemically combined with the saiddessicant, from the bottom hopper region of the preheater to the glassbatch charger 24 of the furnace. The glass batch is thus advanced slowlyand continuously downwardly to the furnace area for melting.

The glass batch in the preheater is indirectly heated by the hot wastegases which are taken from the furnace prior to their arrival at thestack. As shown, the hot gases enter the bottom region of the preheaternear the lower end of the tubes and immediately above lower plate 18,the gases then passing in a serpentine path around the baffle plates 27to the top of the preheater at the underside of upper plate 16, and thenescaping from the preheater through outgoing duct 26. Inlet and outletducts 25 and 26 may be provided with dampers so that the flow of hotgases through the preheater may be accurately controlled. The gasespassing in countercurrent flow to the descending glass formingmaterials, within the tubes, moves between and around the tubes heatingthe same, and the contained glass batch indirectly. Further, the hottestgases thus act upon the hottest portion of the glass formingconstituents in the lower area of the preheater, adding a furtherincrement to their heat before passing into the melting furnace. Asstated hereabove, the hot gas stream may be comprised of waste gasesfrom the furnace heating zone, or preheated combustion air from thefurnace checkers area, or preheated outside air which has beensupplementally heated prior to delivery to the batch preheater.

By proper design of the upper and lower hopper sections of thepreheater, such areas having generally frusto-pyramidal shapes,relatively-uniform and smooth flow of the batch materials by gravitythrough the entire vertical height of the preheater is attained. Thus,flow rates of the batch through all of the heat exchanger tubes, tomaintain the same virtually-full at all times, is obtained from uniformamounts of preheating. The preferred form of construction of thepreheater is having a straight section with rounded corners at an upperregion above the tubes, and a wedge-shaped hopper with rounded cornersat the bottom at the tube lower ends for continuous movement of the hot,dry batch. A sufficient head of batch material is maintained over thetubes to assure such gravity flow, along with a suitable feeder unit toremove preheated material from the bottom of the hopper.

Through proper and thorough mixing of the newly-incoming cold batchfraction, including the dessicant material, uniform and continuousoperation of the preheater apparatus can be practiced. This can beaccomplished when the temperature conditions, and the flow of gases andbatch material, are properly adjusted. Such uniform operation permitsthe maintenance of substantially-constant conditions within thepreheater for delivering significantly-hotter glass batch to thefurnace, greatly increasing the furnace efficiency.

The temperature of the stack gases entering the preheater will vary withfurnace conditions, of course; however, they will normally be from 900°F. to 1100° F. and will frequently average about 1000° F. forsubstantial periods. The gases leaving the preheater will range fromabout 400° F. to 600° F., averaging about 500° F.

Obviously, additional heating means for the preheater may be provided,if desired, although the furnace waste gases are usually fully adequatefor most economical operation. The waste gases, or hot air, normallyenter the preheater at a temperature ranging from about 900° F. to 1100°F., after leaving the furnace combustion or checkers area. As stated,preheated combustion air which has passed through the furnaceheat-recovery area can also be used to heat the batch in the preheater,or a separate supplemental heat source, such as a furner, may be used.The waste gases normally leave the preheater at a temperature of about400° F. to 600° F., to avoid acid condensation problems, while hot aircan be cooled to lower temperatures.

The glass batch mixture usually enters the top of the preheater at aboutambient temperature and leaves the preheater at the valve member 22having a temperature ranging from about 800° F. to 1000° F. Suchtemperatures are possible with a glass furnace which is capable ofmanufacturing about 100 to 300 tons per day of product.

The present invention is not limited to the interaction of one preheaterto one melting furnace, the former being connected with hot gasesleading to one stack. If desired, one preheater may be connected toserve a number of melting furnaces, or a number of preheaters may beassociated with one furnace, and the waste gases emitting therefrom.

The present invention can also be employed to heat individual glassbatch constituents, such as sand, limestone, soda ash, etc., to removemoisture therefrom prior to their introduction into a melting furnace,for example. Further, glass cullet, or mixtures of glass batch andcullet, in widely ranging ratios can also be heated in the apparatus andby the method of this invention, so long as the particulate material hasone or more volatile components therein which tends to condense withinthe heating apparatus. Such batch constituents may be individually orcombinedly heated to temperatures ranging from about 600° F. to 800° F.The glass cullet when heated alone may be heated to even highertemperatures. The mixed batch and cullet can be heated up to a weightpercentage of about 70% cullet or higher, the absorbent dessicantmaterial preventing pluggage of the tubes due to moisture or othervolatile constituent condensation. Virtually all areas of the tubes, andespecially their upper areas, are maintained at a temperature above theboiling point temperature of water, i.e., 212° F. The particulatematerial to be heated can contain a volatile constituent such as wateror a decomposable constituent which produces water on decomposition.Such constituents can be readily eliminated from an upper region of thepreheater by the dessicant material without interrupting the continuousgravity flow of the particulate material.

Various modifications may be resorted to within the spirit and scope ofthe appended claims.

I claim:
 1. The process of preheating pulverous glass batch constituentsprior to their delivery to a glass melting furnace comprising the stepsof introducing the fully-intermixed glass batch constituents into theupper region of a tubular heat exchanger, introducing a dessicantmaterial as a part of said glass batch constituents to absorb thecontained moisture, allowing the glass batch to flow downwardly bygravity through a plurality of open-ended tubes of said heat-exchanger,passing hot gases upwardly through said heat-exchanger around saidopen-ended tubes to heat the glass batch contained therein bycountercurrent indirect heat transfer, and delivering the said heatedglass batch from the bottom of said heat exchanger into the glassmelting furnace.
 2. The process in accordance with claim 1, wherein saiddessicant material comprises calcined lime.
 3. The process in accordancewith claim 1, wherein said dessicant material comprises calcineddolomite.
 4. The process in accordance with claim 1, wherein saiddessicant material comprises a calcium-containing constituent which ispresent in about one-half calcined form and about one-half uncalcinedform.
 5. The process in accordance with claim 1, including the step offully-intermixing the dessicant material into the glass batchconstituents prior to its introduction into the upper region of saidheat exchanger.
 6. The process in accordance with claim 1, wherein saiddessicant material is introduced in an amount in slightly greaterstoichometric amount than the moisture content of the glass batch. 7.The process in accordance with claim 1, wherein said dessicant materialcomprises calcium oxide.
 8. A process for heating a particulate materialsuch as glass batch, individual glass batch constituents, glass cullet,mixtures thereof, and the like, which contain a volatile condensibleconstituent, comprising the steps of introducing the particulatematerial into the upper region of an elongated vertically-mounted hollowenclosed tubular heat exchanger, introducing a dessicant material as apart of said particulate material to absorb the volatile condensibleconstituent, allowing the particulate material to flow downwardly bygravity through a plurality of open-ended hollow tubes of said heatexchanger, passing hot gases upwardly through said heat exchanger aroundsaid open-ended tubes to heat the particulate material contained thereinby countercurrent indirect heat transfer, and delivering the saidparticulate material exteriorly of said heat exchanger in heatedcondition to a point of use.
 9. The process in accordance with claim 8,wherein said dessicant material comprises calcined lime.
 10. The processin accordance with claim 8, wherein said dissicant material comprisescalcined dolomite.
 11. The process in accordance with claim 8, whereinsaid dessicant material comprises a calcium-containing constituent whichis present in about one-half calcined form and about one-half uncalcinedform.
 12. The process in accordance with claim 8, including the step offully-intermixing the said dessicant material into the said particulatematerial prior to their introduction into the upper region of said heatexchanger.
 13. The process in accordance with claim 8, wherein saiddessicant material is introduced in slightly greater stoichometricamount than the moisture content of said particulate material.
 14. Theprocess in accordance with claim 8, wherein said dessicant materialcomprises calcium oxide.